Synthesis, processing and properties of conjugated polymer networks

Christoph Weder*

Received (in Cambridge, UK) 1st July 2005, Accepted 7th September 2005

First published as an Advance Article on the web 12th October 2005

DOI: 10.1039/b509316c

Despite the diverse research activities focused on the chemistry, materials science and physics

of conjugated polymers, the feature of conjugated cross-links, which can provide electronic

communication between chains, has received little attention. This situation may be a direct

consequence of the challenge to introduce such links while retaining adequate processability.

Focusing on recent studies of materials for which charge transport or electrical conductivity data

are available, this feature article attempts to present an overview of the synthesis, processing and

electronic properties of conjugated polymer networks. For the purpose of this discussion, two

distinctly separate architectures—featuring covalent cross-links on the one hand and non-covalent

organometallic bridges on the other—are treated in separate sections. The available data indicate

that cross-linking can have significant benefits for intermolecular charge transfer if the polymers

are carefully designed.

Introduction: why conjugated polymer networks are

of interest

Charge transport in conjugated polymers

Since the discovery of electrical conductivity in p-conjugated

polymers three decades ago,1 semiconducting polymers have

become the focus of major research and development activities

around the globe.2 The excitement for this new generation of

polymeric materials reflects their potential to combine the

processibility and outstanding properties of polymers with the

exceptional, readily-tailored electronic and optical properties

of functional organic molecules. Their potential applications,

especially as synthetic metals,3 and as organic semiconductors

in light-emitting diodes,4 field-effect transistors,5 photovoltaic

cells,6 sensors7 and other devices have motivated the develop-

ment of synthesis and processing methods of conjugated

polymers with unique electronic properties. Breathtaking

progress has been made, and ‘‘plastic electronics’’ technology

has matured beyond the onset of commercial exploitation8 of

conjugated polymers into a variety of applications that range

from corrosion control9 to light-emitting diodes.4 One key

problem for the full technological exploitation of polymer

semiconductors, however, is that they display generally a much

lower charge carrier mobility, m, than inorganic materials,10

and hence also a decreased electrical conductivity s (which is

proportional to m). This limitation is related to the fact that

the charge transport in conjugated polymers is a function of

intra-chain charge diffusion and inter-chain interactions, i.e.

hopping.11 The charge carrier mobility in these materials

is usually limited by disorder effects, which prevent

efficient inter-chain coupling and lead to materials with one-

dimensional electronic properties.12–15 Exciting progress

has been documented for polymers with high degrees of

supramolecular order, and in some cases orientation.16–22 For

example, disordered, amorphous samples of poly(3-alkyl-

thiophene)s (PATs) display a hole mobility in the order

of y1025 cm2 V21 s21; this value is increased up to

y0.2 cm2 V21 s21 in semi-crystalline films of PATs in which

p-stacked conjugated polymer lamellae are organized parallel

to a substrate and allow for highly efficient in-plane charge

transport.16,17,21,22 At the same time, a significant improve-

ment is observed in the electrical conductivity. For example,

the electrical conductivity of iodine-doped poly(2,5-

dimethoxy-p-phenylene vinylene) fibers was shown to increase

from 20 to 1200 S cm21 upon uniaxial orientation by tensile

deformation.23 In another exemplary study, Sirringhaus et al.

exploited the liquid crystalline (LC) character of a 9,9-

dioctylfluorene-bithiophene copolymer.19 In this case, the LC

polymer was uniaxially oriented with the help of an alignment

layer, and the polymer was quenched into a nematic glass that

displayed significantly enhanced carrier mobilities of up to

0.02 cm2 V21 s21 along the alignment direction. Thus, the

process of ordering/orienting conjugated polymers indeed

affords materials with significantly improved charge carrier

Department of Macromolecular Science and Engineering, Case WesternReserve University, 2100 Adelbert Road, Cleveland, OH 44106-7202,USA. E-mail: christoph.weder@case.edu; Fax: (+1) 216 368 4202;Tel: (+1) 216 368 6374

Christoph Weder is Associate Professor of MacromolecularScience and Engineering at Case Western Reserve University inCleveland, Ohio. Weder was educated at the Swiss FederalInstitute of Technology (ETH) in Zurich where he earned hisacademic degrees from the Departments of Chemistry (Dipl.Chem.) in 1990 and Materials (Dr. sc. nat.) in 1994. After anappointment as a postdoctoral fellow at the MassachusettsInstitute of Technology, Weder returned to ETH in 1995 whenthe Department of Materials appointed him firstly as head-assistant and lecturer, and then in 1999 after completion of his‘Habilitation’, as an independent lecturer. He moved to Case in2001, where he established the Functional Polymer Laboratory.Weder’s primary research interests are the design, synthesis andinvestigation of the structure–property relationships of novelfunctional polymers, in particular, materials with advanced opticor electronic properties.

FEATURE ARTICLE www.rsc.org/chemcomm | ChemComm

5378 | Chem. Commun., 2005, 5378–5389 This journal is � The Royal Society of Chemistry 2005

mobility and electrical conductivity. It should be noted,

however, that many of the processing protocols employed

for the fabrication of materials with a high degree of order and

orientation24 are incompatible with the preferred low-cost pro-

cesses of plastic electronic manufacturing, for example spin-

coating,25 inkjet26 and screen printing.27 On the other hand,

there are important exceptions to this notion; a prominent

example is the inkjet printing of the aforementioned thermo-

tropic LC materials, which has allowed the fabrication of all-

polymer transistors with appreciable device characteristics.26a

An orthogonal approach for improved charge transport

The introduction of p-conjugated cross-links between con-

jugated macromolecules represents an attractive alternative

approach for the designing of semiconducting polymers with

improved charge transport characteristics.28 Indeed, in an ideal

p-conjugated macromolecular network that features conju-

gated cross-links (Fig. 1), intra-chain diffusion may become the

predominant mechanism for charge transport, while inter-

chain processes—if at all—only play a subordinate role. An

important prerequisite for this mechanism is that the electronic

potentials of the cross-links (i.e., HOMO and LUMO or

electron affinity and ionization potential) match those of the

linear segments, so that these moieties do not serve as traps or

barriers for the charge carriers, but rather allow for adequate

electronic coupling. As shown in Fig. 1, the networks can

be designed to rely on either covalent or non-covalent

interactions. The first case is based on the introduction of a

conjugated tri-functional (or higher functionalized) monomer

along with the conventional bi-functional monomers

(Fig. 1a, left). Obviously, this approach ultimately leads to

an intractable polymer network, which has to be processed

prior to or during network formation. A variation of this

strategy is a two-stage process, in which linear precursor

macromolecules with cross-linkable functionalities are first

prepared, processed and subsequently cross-linked (Fig. 1a,

right). Networks based on physical cross-links (e.g., hydrogen

bonds, electrostatic interactions or chain entanglements)

represent architectures in which non-covalent interactions

lead to potentially very useful properties,28b but the exact

nature and influence of the cross-links is often ill-defined

and makes the elucidation of structure–property relationships

difficult. Therefore, the present review emphasizes the

important class of organometallic networks that are formed

through coordination bonds between ligand sites comprised in

the organic semiconductor and metallic cross-links (Fig. 1b).

These metallopolymers are also intractable but are accessible

either via ligand-exchange reactions (Fig. 1b, left) or,

alternatively, by the polymerization of pre-fabricated ligand–

metal complexes (Fig. 1b, right).

Scope of this review

Interestingly, despite the diverse research activities focused on

the chemistry, materials science and physics of conjugated

polymers, the feature of conjugated cross-links has received

little attention, at least as far as systematic studies and well-

defined materials are concerned. This situation may be a direct

consequence of the challenge of introducing such cross-links

and retaining adequate processability. On the other hand, in

many cases, the exact structure of the cross-linked semi-

conducting polymers is not known. While conjugated polymer-

based networks featuring non-conjugated cross-links based on

covalent29 or non-covalent bonds30 have been deliberately

prepared and studied by a number of research groups,

examples of cross-links that might allow adequate electronic

transport between chains are rather rare, and in many cases

have been obtained serendipitously and/or lack unambiguous

characterization. Focusing on selected recent examples of

materials for which charge transport or electrical conductivity

data are available, and whose chemical structure has been

appropriately established, this review attempts to present a

concise overview of the synthesis, processing and electronic

properties of conjugated polymer networks. For the purpose of

discussion, two distinctly separate architectures—featuring

covalent cross-links on the one hand and non-covalent

organometallic bridges on the other—are treated in separate

sections. The available data indicate that cross-linking can

have significant benefits for intermolecular charge transfer if

the polymers are carefully designed.

Networks based on organometallic cross-links

Electronic communication between metal and polymer

The general approach of introducing transition metals into

conjugated polymers has received considerable attention, in

Fig. 1 Simplified schematic representation of cross-linked conjugated

polymer networks with covalent (a) and non-covalent organometallic

cross-links (b). In the case of covalent networks, one-step (left)

and two-step protocols (precursor approach, right) are commonly

employed. Organometallic networks can be prepared by ligand-

exchange reactions (left) or the polymerization of a pre-fabricated

ligand–metal complex (right).

This journal is � The Royal Society of Chemistry 2005 Chem. Commun., 2005, 5378–5389 | 5379

particular due to the potential of manipulating the electronic

properties of these materials.31–36 Conventional concepts for

the design of p-conjugated metallopolymers rely on either the

incorporation of metal centers into the polymer backbone,

their coordination to the conjugated backbone, or their

attachment via conjugated or non-conjugated spacer units in

the form of side groups. The different mechanisms of electrical

conduction in metallopolymers have recently been discussed in

a scholarly manner by Swager and Holliday.37 Classic electron

transfer theory38 distinguishes two different situations—outer

and inner sphere transfer—depending on the electronic

coupling between the orbitals of the transition metal and

those of the conjugated macromolecules. In the case of outer

sphere transfer, the metal and delocalized polymer orbitals

lack significant mixing (systems in which the metal is attached

to the conjugated polymer backbone via a non-conjugated

spacer typically fall into this category), and as a result, the

transition metals may not be intimately involved in the overall

charge transport. By contrast, inner sphere transfer, which is

of interest here, is observed for systems with strong overlap

between the orbitals of the metal and conjugated macro-

molecules.39 This can be the case if the metal centers form part

of the polymer backbone or coordinate directly with the latter.

Importantly, a matching of the energies of the involved

orbitals (macroscopically manifested by matched redox

potentials or valence and conduction bands) is important

for efficient transport through the polymer–metal complex;

this mechanism is also referred to as superexchange.33,40

Mismatched energies, in contrast, can deteriorate the charge

transport, since charge localization caused by the metal centers

may lead to charge trapping.

Networks prepared by ligand-exchange reactions

As mentioned heretofore, cross-linked organometallic poly-

mers are intractable, i.e. non-melting and insoluble materials.

Ligand-exchange reactions between a linear conjugated poly-

mer that comprises adequate ligand sites and a metal complex

with, ideally weakly bound, low-molecular weight ligands

represents one important possibility for preparing thin films

of these polymers (Fig. 1b, left). Another possibility is the

polymerization of pre-fabricated ligand–metal complexes

(Fig. 1b, right). One of the earlier examples of the formation

of organometallic conjugated networks by the ligand-exchange

approach was reported by Wright.41 His experiments sug-

gested that upon thermal treatment or UV irradiation of

solid thin films of poly(arylene ethynylene)s containing the

Cr(CO)3–benzene moiety, cross-linking occurred upon loss

of CO with the formation of phenylene–Cr(CO)2–ethynyl

moieties (1, Scheme 1). Speculating that multi-coordination

permits electronic communication between the metals through

the p-conjugated chains, Hirao et al. described, among

other systems,42 the synthesis of organometallic networks of

poly(o-toluidine) with Pd2+ or Cu2+ coordinating to the imine

moieties in the polymer (2, Chart 1).43 Unfortunately, the

electronic properties of these polymers have remained largely

unexplored.

In another study, we demonstrated that the unsaturated

carbon–carbon bonds in the backbone of poly(p-phenylene

ethynylene)s (PPEs) can be utilized as quite a versatile binding

motif.44–46 The conjugated polymers employed were the

alkoxy-substituted PPEs, 3,47,48 which are representative of

this family of conjugated polymers with well-documented

optoelectronic properties,49 and offer two ethynylene moieties

per repeating unit as potential ligand sites (Scheme 2). In the

initial experiments dinuclear [Pt-(m-Cl)Cl(PhCHLCH2)]2 (4)50

was employed as the cross-linker. The ethynylene groups

comprised in the PPE were shown to readily coordinate to Pt2+

in exchange for weakly-bound styrene ligands.44 An extensive

in situ 195Pt NMR study revealed that in dilute CHCl3solutions the equilibrium of the investigated PPE–Pt systems

dictates non-cross-linked structures (Scheme 2, 5, z # 0).

Importantly, under these conditions, the system remains

homogeneous and therefore processible. Evaporation of the

solvent leads to a shift of the equilibrium to PPE–Pt network

structures (Scheme 2, 5, z . 0), and due to its volatile nature,

the liberated styrene ligand is also removed during this

process.44 Spin-coating resulted in films of good optical

Scheme 1 Cross-linking reaction proposed to occur in solid thin films

of poly(arylene ethynylene)s containing the Cr(CO)3–benzene moiety

upon heating.41

Chart 1

Scheme 2 Ligand-exchange reaction between PPE 3a and [Pt-(m-Cl)-

Cl(PhCHLCH2)]2 (4), leading to cross-linked metallopolymers 5.44

5380 | Chem. Commun., 2005, 5378–5389 This journal is � The Royal Society of Chemistry 2005

quality that were unequivocally cross-linked. As expected, the

coordination of Pt2+ markedly influences the photophysical

characteristics of the PPE; the photoluminescence is quenched

upon complexation, and at high Pt contents, the absorption

maximum experiences a hypsochromic shift. Clearly, the

Cl-bridged dinuclear cross-links originally employed cannot

be expected to provide significant p-conjugation between

chains. Indeed, time-of-flight (TOF) measurements conducted

as a function of carrier type, electric field, sample thickness

and Pt content51 suggest that the photocurrents observed for

thin films of 5 (3b–Pt2+) are range-limited, indicating trapping

of both electrons and holes in this material. In earlier work on

linear Pt2+-containing poly-ynes, p-conjugation was found to

be preserved through the metal atom; however the hybridiza-

tion between the p-orbitals of the polymer ligand and the

platinum 5d orbitals was found to be weak.52

In subsequent experiments, Pt0 was chosen as the cross-

linker, since it forms stable bis(ethynylene) complexes,53 which

due to the interaction of the p bond of the ligand with the dx22y2

orbital of the Pt or via p-backbonding from the dxz orbital of

Pt to p* orbitals of the ligands, may allow for electronic con-

jugation.52,54 A styrene solution of Pt(styrene)3 (6) served as

the Pt0 source,55 and model reactions with diphenylacetylene

(DPA, 7) confirmed that even in the presence of a y150-fold

excess of styrene, the ligands of 6 are quantitatively replaced

by 7, and the only product formed is Pt(DPA)2 (Scheme 3,

8).56 The analogous reaction between PPE 3b and 6 was

accomplished by combining styrene solutions of these reac-

tants (Scheme 3);45,46 the ratio of the molar concentrations

of Pt0 and phenylene ethynylene (PE) moieties, [Pt0]/[PE], was

varied between 0.016 : 1 and 0.34 : 1; in the following such

ratios are expressed as single numbers, e.g. 0.016 and 0.34.

Spin-coating and solution casting yielded homogeneous thin

films of the cross-linked metallopolymer 9 (Scheme 3). The

carrier mobility of a series of metallopolymers 946 and the

uncomplexed PPE 3b57 was determined by TOF measurements

as a function of carrier type, electric field and Pt0 content.

The shape of the photocurrent transients of 3b and 9

([Pt0]/[PE] 5 0.17), shown in Fig. 2, is characteristic of

dispersive transport.58 This mechanism is typical for materials

with a high degree of spatial and/or energetic disorder, and

is concomitant with a wide variation of local transport

rates.59 High electron (1.9 6 1023 cm2 V21s21) and hole

(1.6 6 1023 cm2 V22 s21) mobilities were found at low electric

field strength (3.8 6 104 V cm21) for the neat 3b. The data

shown in Fig. 2 and Fig. 3 demonstrate that the carrier

mobility strongly increases upon introduction of Pt0. A distinct

enhancement of the mobility was observed for 3b–Pt0 with a

small [Pt0]/[PE] value, but the effect levels off at a [Pt0]/[PE]

ratio of y0.17 when charge carrier mobilities of 1.6 61022 cm2 V21 s21 (electrons) and 1.4 6 1022 cm2 V21 s21

(holes) are reached. These values are an order of magnitude

higher than those of the neat PPE. Interestingly, the enhance-

ment is similarly pronounced for electron and hole transport;

thus the metallopolymers 9 are very effective ambipolar

semiconductors. The charge carrier mobility of polymers 9

was found to decrease with increasing bias (Fig. 3). This

behavior is consistent with a hopping transport model that

accounts for off-diagonal (positional) disorder caused by

variations in the inter-site distances, in addition to diagonal

(energetic) disorder in the transport manifold.60 The large

Scheme 3 Ligand-exchange reaction between Pt(styrene)3 (6)

and PPE 3b or diphenylacetylene (7), leading to cross-linked metallo-

polymers 9 and model compound bis(diphenylacetylene)platinum (8),

respectively.46,56

Fig. 2 Electron TOF photocurrent transients of PPE 3b (solid line,

film thickness L 5 8 mm) and metallopolymer 9 (dotted line, L 5 30 mm,

[Pt0]/[PE] 5 0.17) films in linear (top) and logarithmic (bottom)

plots, measured at a temperature of 295 K and an electric field of

1.5 6 105 V cm21. Reproduced with permission from Ref. 46.

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off-diagonal disorder results in a negative field dependence

of the mobility at low fields, because a stronger field favors

forward hopping and inhibits faster routes for carriers

involving hops transverse to the applied electric field.

With the notion that other examples of suitable bis-(g2-

diphenyl ethynylene)metal complexes, which could provide

electronic conjugation between chains, are rare,61 2,29-bipyr-

idine (Bipy) moieties were introduced as auxiliary ligands into

the backbone of PPEs.62 This versatile ligand63 has already

been introduced into a plethora of macromolecules that form

the basis of a number of metallo-supramolecular systems.34

Pioneering work on PPEs with Bipy groups in the polymer

backbone and linear metal complexes of these polymers has

been carried out by the groups of Schanze64 and Klemm.65

Interestingly, the metal-complexed PPEs investigated in these

studies were almost exclusively prepared by polymerizing

metal-complexed monomers, rather than by complexation of

the Bipy-containing polymer with metals. However, the latter

framework, which—mainly with sensor applications in mind

and not under consideration of potential network formation—

has been applied by a number of groups for a variety of other

conjugated polymers,66–71 is formidably suited to preparing

metallo-supramolecular PPE networks. Systematic ligand-

exchange reactions were conducted with PPEs containing

different fractions of the Bipy moiety (BipyPPEs 10, 11) and a

variety of metal complexes (Scheme 4).62 For example,

BipyPPE–Cu+ networks were prepared via the complexation

of 10 (a copolymer featuring phenylene ethynylene and

bipyridine moieties in an alternating fashion) with

[CuI(CH3CN)4]PF6. UV-vis absorption and photolumines-

cence (PL) emission spectra, acquired upon titrating 10 with

[CuI(CH3CN)4]PF6 in CHCl3–CH3CN (15 : 1 v/v), are shown

in Fig. 4. The intensity of the characteristic p–p* transition

around 423 nm, associated with the conjugated backbone of

10, systematically weakened upon addition of Cu+, and a new

band at ca. 452 nm developed that was interpreted as being

due to a metal-to-ligand charge transfer complex.62 As can be

seen from the inset in Fig. 4a, the intensity of the transition at

452 nm steadily intensified with increasing [Cu+] : [Bipy] ratio,

before levelling off at a [Cu+] : [Bipy] ratio of about 0.5.

Similarly, the polymer’s PL was gradually quenched upon

Fig. 3 Electron mobility of metallopolymer 9 as function of [Pt0]/[PE]

and electric field ([Pt0]/[PE]: % 5 0, m 5 0.016, $ 5 0.086, n 5 0.17,

& 5 0.25, # 5 0.34). Reproduced with permission from Ref. 46.

Scheme 4 Schematic representation of the formation of metallo-

supramolecular networks through the complexation of 2,29-bipypri-

dine-containing poly(2,5-dialkyloxy-p-phenylene ethynylene)s 10 and

11 with transition metals.

Fig. 4 UV-vis absorption (top) and PL emission (bottom) spectra

acquired upon addition of tetrakis(acetonitrile)CuI-hexafluorophos-

phate to BipyPPE 10 (concentration of polymer-bound Bipy 5 1.93 61025 M) in CHCl3–CH3CN (15 : 1 v/v). Shown are spectra at selected

[Cu+] : [Bipy] ratios of 0 (—), 0.09 (&), 0.19 ($), 0.28 (m), 0.38 (.),

0.48 (r), 0.57 (%), 0.76 (#), 0.96 (n) and 1.92 (,). The insets show

the absorption at 452 nm (a) and the emission at 459 nm (b) as a

function of [Cu+] : [Bipy] ratio.

5382 | Chem. Commun., 2005, 5378–5389 This journal is � The Royal Society of Chemistry 2005

addition of [CuI(CH3CN)4]PF6 (Fig. 4b). Scatchart plots of

the data presented in Fig. 4 are characteristic of positive

cooperative binding,72 and the observed changes in the UV-vis

and PL spectra were fully reversible upon addition of a

competing ligand, such as free bipyridine, to the system. Thus,

these results are consistent with the (reversible) formation of

BipyPPE–Cu+–BipyPPE cross-links between the conjugated

macromolecules and point to relatively large binding con-

stants. The fact that changes in the optical spectra level off at a

metal–ligand ratio of 0.5 clearly indicates the formation of 2 : 1

ligand–metal complexes, which in turn suggests the formation

of well-defined network structures. The fact that the partially

metallated polymer retained a significant extent of PL emission

(Fig. 4b) is indicative of a limited exciton migration along the

polymer to the non-radiative low band gap sites. This feature

appears to be related to the ‘‘de-conjugated’’ nature of

uncomplexed, twisted Bipy moieties that cause ‘‘optical

insulation’’ and allow the coexistence of multiple chromo-

phores on the same macromolecule. Their weak electronic

coupling is in marked contrast to the PPE-based polymer

systems reported by Swager and co-workers, which act as

‘‘molecular wires’’ and display energy migration through

conjugated segments that comprise up to y50 repeat units.73

The complexation of BipyPPEs 10 and 11 with the perchlo-

rates of Co2+ and Ni2+ led to very similar optical changes to

those found in the case of Cu+. Interestingly, the addition of

Zn(ClO4)2 or Cd(ClO4)2 caused a somewhat more pronounced

change to the absorption band than did Cu+, Ni2+ or Co2+, and

in the case of both metals, broad structure-less emission bands

centered at 619 (Zn2+) and 591 nm (Cd2+) developed. These

results reflect the fact that Zn2+ and Cd2+ both exhibit a fully

occupied d-orbital (Zn2+: 3d10, Cd2+: 4d10) that frequently

displays a weak tendency for the formation of metal-to-ligand

charge transfer.74 Hence, the complexation of these metals

with Bipy-containing polymers does not usually lead to MLCT

complexes.70 Rather, the optical changes appear to be related

to a significant reduction of the polymers’ p–p* transition on

account of a planarization of the Bipy moiety,67,68 as well as

an electron density variation upon complexation with the

electron-poor metals.70 In view of the fully occupied d-orbital

of the metal, the observed emission cannot be related to a d–d

transition, but appears to be caused by intra-ligand p–p*

transitions.

Networks prepared by polymerization of pre-fabricated metal–

ligand complexes

The polymerization of pre-fabricated metal–ligand complexes

(Fig. 1b, right) represents another framework for the synthesis

of cross-linked metallopolymers. If no co-monomer is

employed as a linear chain extender, the cross-link density of

the resulting materials is usually very high. As will become

evident from the examples presented here, virtually all

materials synthesized by this approach were prepared by

electrochemical polymerization; the thiophene (or oligothio-

phene) moiety, which can usually be polymerized by electro-

chemical means through oxidative coupling at the a (strongly

favored) or b position,35 has been the most popular motif as

far as the organic conjugates segment is concerned. It should

be noted that the electrochemical polymerization method is a

very convenient general methodology that allows the facile

preparation of laboratory-scale thin films of high quality, but

its usefulness appears to be more limited when it comes to the

commercial production of electronic polymers.

Swager’s group has reported the investigation of a series of

polythiophene–Ru(bpy)3 hybrid materials.75 These polymers

were synthesized by the electrochemical polymerization of

Ru(bpy)3 derivatives that were appended with bithienyl

moieties (e.g., 12, Chart 2). The choice of Ru(bpy)3 centers

as the redox component is the result of the broad manifold of

reversible redox processes associated with this type of complex

that make redox matching with the polymer likely. On the

other hand, the electrochemical polymerization of the bipy-

bridged bithienyl monomers was found to proceed smoothly

for both the free ligand as well as the metal complex.

Comparative experiments led to the conclusion that cross-

linking in these polymers is an important contributor to

high conductivity. Indeed, the highest electrical conductivity

(3.3 6 1023 S cm21, determined as in many of studies reviewed

in this section, by in situ conductivity experiments76) was

reported for poly-12 (Chart 2).75b The cyclic voltammograms

of poly-12 display both metal-centered and thiophene-based

electroactivity, and similarly high redox conductivities were

observed for the thiophene-based oxidation and metal-based

reduction processes. Poly-12 is highly cross-linked and, in

contrast to other members of the investigated series, possesses

a 4,49-substitution pattern of the Bipy moiety that allows

effective orbital overlap between the polythiophene segments

and the dxz and dyz orbitals of the ruthenium centers, and

therefore electronic transport through the organometallic

segments.

Wolf et al.35,77 studied a series of polythiophenes that

were cross-linked via different Pd complexes. In this case, the

approach relied on the electropolymerization of monomers

13–15 (Chart 3), in which 39-diphenylphosphino-2,29:5920-

terthiophene moieties were coordinated in three different

modes with the metal. All three monomers could be poly-

merized to yield thin films that displayed an in situ conduc-

tivity of between 1024 (poly-15) and 1023 S cm21 (poly-13)

when oxidized. Based on comparative studies with analogous

monomers, in which one or both of the terthiophene’s

a-positions were blocked with methyl groups and—where

possible—oligomers thereof, the authors concluded that the

role of the metal is largely inductive in case of poly-13, which

features a dinuclear Pd complex. Charge transport in this

material presumably results from delocalization along the

extended polythiophene chains and p-stacking, rather than

Chart 2

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through the metal cross-links. On the other hand, the

conductivity is thought to involve a contribution from cross-

metal delocalization in the case of poly-15, but the effect

appears to be rather small.

The groups of Pickup78 and Skabara79 have electropoly-

merized bis[1,2-di(2-thienyl)-1,2-ethenedithiolene]nickel78 (16)

and bis[(terthiophene)dithiolene]complexes79 containing Ni2+,

Pd2+ or Au3+ (17) (Chart 4). The conductivities of this series of

metallopolymers, determined by impedance spectroscopy, are

in the range of ca. 1026 to 1025 S cm21 in a potential range of

0 to +1 V, and around 1024 S cm21 (poly-16) in a potential

regime where the polymer is oxidized. Films of poly-17 show

only one redox wave for the metal dithiolene unit, which is less

reversible than in the monomer, suggesting that there is little

electronic communication between adjacent metal units.78

Vidal et al. investigated polythiophenes that comprised

the 1,10-phenanthroline moiety as a ligand.80 Entwined

architectures around copper ions were obtained by dimerizing

monomers 18 and 19 by complexation with Cu+ and

electropolymerization of the entwined intermediates 20 and

21 (Scheme 5). Interestingly, electrochemical studies coupled

with in situ conductivity experiments and X-ray absorption

spectroscopy revealed rather different electronic properties for

poly-20 and poly-21. In their oxidized states, poly-18 and poly-

20 display similar conductivities, in the order of 1024 S cm21.

The experiments clearly demonstrated that the conductivity of

poly-20 is related to transport through the conjugated organic

segments and that no significant electronic interactions

between the metal and the polymer occur. The case of poly-

21 clearly contrasts with that of poly-20. The cyclic voltam-

mograms (CVs) of poly-21 suggest a mixing of the redox

processes associated with the copper and the conjugated

organic parts. In situ conductivity experiments revealed a

stable potential window of high conductivity that corresponds

to the oxidation of the polymer; the level of conductivity (9 61024 S cm21) was found to be an order of magnitude higher

than that of poly-18, poly-19 or poly-20. De-metallated films

of poly-21 showed a significant decrease in conductivity. Thus,

the work nicely demonstrates charge transport between chains

through the copper centers.

Networks based on conjugated covalent cross-links

Precursor approach

Conjugated polymer networks with covalent cross-links can

be synthesized and processed into the scientifically but also

technologically-relevant shape of thin films (and other shapes),

using different strategies. One approach is the so-called

precursor approach, a two-stage process, in which well-defined

linear precursor macromolecules with cross-linkable function-

alities are first prepared, processed and subsequently cross-

linked (Fig. 1a). Some of the earlier comprehensive studies on

the synthesis and characterization of ‘‘hypercross-linked’’

conjugated polymers by this approach were reported by the

groups of Whitesides81 and Stille.82 Their work was based on

low-molecular weight conjugated precursor polymers and

oligomers comprising thermally cross-linkable diacetylene

groups and a variety of different aromatic moieties. As far as

electronic transport is concerned, the most comprehensive data

sets are available for the poly(2,5-ethynylenethiophene)s, 22,

and the related materials 23 and 24 shown in Scheme 6.82

Derivatives of 22, in which the aromatic moiety was

additionally derivatized with alkyl chains, were soluble

and could be appropriately characterized and processed

into thin films by spin-coating or solution casting. Thermal

treatment allowed for solid state cross-linking at moderate

temperatures (150–200 uC). Based on 13C CP-MAS studies, the

principal structure of the resulting cross-links was identified as

Chart 3

Chart 4

Scheme 5 Schematic representation of the formation of entwined

precursors via dimerization of 18 and 19 through complexation

with Cu+.

5384 | Chem. Commun., 2005, 5378–5389 This journal is � The Royal Society of Chemistry 2005

a diene-containing material (Scheme 6). The intrinsic con-

ductivities of the un-doped linear (i.e. not cross-linked)

polymers 22–24 were in the range 10213–10211 S cm21, i.e.,

at the lower end of the semiconducting regime. Doping

with iodine led to a modest increase in conductivity (10211–

1028 S cm21), while the use of arsenic pentafluoride, which is

a stronger oxidant than I2, afforded semiconducting materials

with conductivities in the range of 1028–1026 S cm21. Rather

interestingly, the conductivities of the cross-linked products of

22–24 in their un-doped states varied over several orders of

magnitude; in some cases, the cross-linked product displayed a

significantly higher conductivity than the un-reacted parent

(e.g., 22a, 1028 vs. 10213 S cm21). This behavior is consistent

with the formation of defects upon thermal cross-linking that

may act as charge carriers. On the other hand, the doping of

cross-linked materials with AsF5 did not increase their

conductivity beyond the values observed for the similarly-

doped linear polymers. This result was explained by the lack of

interaction between the polymers and the dopant due to a

relatively high oxidation potential of the polymer on the one

hand, and the potential inability of the counterion (AsF62) to

become incorporated in the polymer matrix on the other. The

data, unfortunately, do not allow a conclusion to be drawn

about whether or not the cross-linking imparts the charge

transport between chains.

Another example for the precursor approach comes from

Lavastre et al., who reported the formation of conjugated

polymer networks through the heat treatment of poly[(4-

ethynyl)phenylacetylene] (Scheme 7).83 The cross-linking reac-

tions were studied via thermoanalytical techniques (DSC and

TGA) and the resulting insoluble products characterized by

means of infrared spectroscopy. Based on the IR data and by

comparison with earlier work, the generation of ene–yne

fragments was suggested as the result of the cross-linking

reaction. Unfortunately however, no charge mobility or

conductivity data have been reported for this system.

Conjugated polymer networks by one-step protocols

A variety of protocols have been employed for the preparation

of cross-linked conjugated polymer thin films by one-step

protocols, including electrochemical methods and the proces-

sing of dispersions. In an important study, Joo et al. have

compared the electronic characteristics of different polypyrrole

samples that were synthesized electrochemically and chemi-

cally, and feature different degrees of conjugated side chains

and/or cross-links (25, Chart 5).84 X-ray photoelectron

spectroscopy suggests that a significant fraction of the pyrrole

units not only react in the 2,5 positions to form linear

macromolecules, but that side reactions in the 3-position lead

to branching or cross-linking between chains (Chart 5). While

the analytical techniques employed in this study did not

allow an unambiguous discrimination to be made between

(originally unintentionally introduced) side chains and cross-

links, a clear difference between the investigated samples was

evident; about 20% of the pyrrole moieties of chemically

prepared, dodecylbenzenesulfonic acid (DBSA)-doped poly-

pyrrole were incorporated in side chains or cross-links, while

that fraction was increased to about 33% in the case of

electrochemically prepared, PF62-doped polypyrrole. In a

systematic study, the authors have related these structural

differences to the electronic properties of these polymers.

For chemically synthesized polypyrrole samples that were

doped with DBSA or naphthalenesulfonic acid (NSA) the dc

conductivity was ¡0.1 S cm21 at room temperature, and their

temperature dependence displayed a strong localization

behavior. By contrast, the dc conductivity of electrochemically

synthesized polypyrrole doped with hexafluorophosphate

(PF62) was in the critical or even metallic regime (50 S cm21)

and displayed a much higher density of states than the

chemically synthesized samples. Thus, the highest conduc-

tivities were found for the material (electrochemically pre-

pared, PF62-doped polypyrrole) for which the highest density

of cross-links and side chains was observed. The results suggest

improved inter-chain interactions for this system and agree

with the expectation of percolation of the metallic state with

increasing cross-link density.

Scheme 6 Formation of diene-containing, hypercross-linked net-

works by thermal cross-linking of poly(2,5-ethynylenethiophene)s 22

and related materials 23 and 24.82

Scheme 7 Cross-linking reaction proposed to occur in poly[(4-

ethynyl)phenylacetylene] upon thermal treatment.83 Chart 5

This journal is � The Royal Society of Chemistry 2005 Chem. Commun., 2005, 5378–5389 | 5385

We recently embarked on the synthesis of poly(p-phenylene

ethynylene)49 networks with covalent cross-links.85,86 These

polymers (26) were synthesized by the palladium-catalyzed

cross-coupling polycondensation of 1,4-diiodo-2,4-dialkoxy-

benzenes (27), 1,4-diethynyl-2,5-bis-(octyloxy)benzene (28)

and various quantities (ratio of 27 : 29 5 0.1–10) of 1,2,4-

tribromobenzene (29) as a cross-linker (Scheme 8).85 The

reaction may allow linear PPE segments of appreciable

molecular length to grow before cross-linking, because the

reactivity of the aryl bromide in the cross-coupling reaction is

lower than that of the aryl iodide.87 If the polymerization was

carried out under conventional reaction conditions (i.e. in

homogeneous toluene–diisopropylamine solutions), the reac-

tion mixtures gelled after a relatively short reaction time.

Consistent with the anticipated network structure, the

products thus prepared did not dissolve but swelled signifi-

cantly (between ca. 300–600% w/w) in chloroform and toluene,

both of which are good solvents for the linear polymer. These

conjugated polymer networks were highly luminescent when

swollen and their photoluminescence spectra were very similar

to those of their parent linear PPE. As mentioned heretofore,

the potential usefulness of the cross-linked polymers under

investigation in actual devices depends on the ability to process

these materials into thin films (and possibly other shapes). One

approach to accomplishing this objective follows the general

framework routinely employed for standard thermoset poly-

mers, and is based on the simultaneous polymerization and

processing of the material into the desired shape. Indeed, it was

shown that cross-linked coherent thin films can be produced

by casting the reaction mixture and conducting the poly-

merization reaction while shaping the object.85 An alternative

to overcoming the problem of processing is to synthesize the

cross-linked target polymers in the form of spherical particles

that can be processed from (aqueous) dispersions. By applying

concepts employed for the preparation of dispersions of linear

conjugated polymers88 and exploiting the fact that some metal-

catalyzed cross-coupling reactions are tolerant to the presence

of water,89 it was shown that cross-linked conjugated polymer

particles can be conveniently produced by polymerization in

aqueous emulsions.86 The size of the resulting particles could be

readily tuned over a wide range (nm to mm) by modifying the

reaction conditions (Fig. 5). For example, micrometer-sized

particles were obtained by carrying out the polymerization of

monomers 27–29 in a vigorously-stirred water–toluene–diiso-

propylamine mixture, utilizing sodium dodecyl sulfate (SDS)

as a surfactant. The mixture formed an oil-in-water emulsion

and most of the reactants and catalysts were presumably

dissolved in the organic phase. The polymer produced was

precipitated and isolated as a dry powder, but the product

could readily be re-dispersed into well-separated particles by

ultrasonication in solvents such as toluene (without further

surfactant addition), as shown by the micrographs in Fig. 5. As

can be seen from Fig. 5, the size distribution of the polymer

particles produced was relatively narrow, with an average

diameter of y4.7 mm. The chemical composition of the

polymer was comparable to that of the homogenous reaction

product, and elemental analysis revealed that the SDS content

of the final product was very low.86 To further reduce the

average particle size, the polymerization reaction was con-

ducted under the emulsion conditions outlined above, but with

an ultrasonic bath employed instead of a mechanical stirrer

and the concentration of the surfactant was increased.

Scanning electron microscopy pictures (Fig. 5) confirm that

Scheme 8 Synthesis of cross-linked PPEs 26 by the palladium-

catalyzed cross-coupling reaction of 1,4-diiodo-2,4-dialkoxybenzenes

(27), 1,4-diethynyl-2,5-bis-(octyloxy)benzene (28) and various amounts

(ratio of 27 : 29 5 0.1–10) of 1,2,4-tribromobenzene (29). 26a, 27a:

R1 5 2-ethylhexyl, R2 5 Me; 26b, 27b: R1 5 R2 5 2-ethylhexyl.85,86

Fig. 5 Photograph (a), optical micrograph (b) and scanning electron

micrograph (c) of cross-linked conjugated milli- (a) micro- (b), and

nanoparticles (c) prepared by emulsion polymerization according to

Scheme 8. Photographs and optical micrographs were taken in

fluorescence mode under excitation at 366 nm and in transmission/

reflection mode, with the polymer particles dispersed in toluene. (a)

and (b) are reproduced with permission from Ref. 86.

5386 | Chem. Commun., 2005, 5378–5389 This journal is � The Royal Society of Chemistry 2005

cross-linked nanospheres with a diameter between ca. 50 and

400 nm and a narrow size distribution can be produced by this

method. The resulting polymer particles were processed into

homogeneous thin films by casting dispersions of these

materials in toluene.51 It appears that this general approach

is universally applicable to many polymer systems. Preliminary

TOF measurements revealed a charge carrier mobility of ca.

7 6 1023 cm2 V21 s21 for holes and 9 6 1023 cm2 V21 s21 for

electrons at low electric field strength (3.8 6 104 V cm21) for

polymer 26b.51 These values are significantly higher than

those of the linear polymer 3b at the same field strength (1.6 61023 cm2 V22 s21 for holes and 1.9 6 1023 cm2 V21 s21 for

electrons), suggesting that the cross-linking gives rise to

improved inter-chain interactions for this system.

Concluding remarks

In view of the fact that intermolecular charge transport has

long been recognized as an important factor for the overall

conductivity of conjugated polymers, and with the notion that

much of the early work on these materials was directed at

developing a fundamental understanding of the structure–

property relationships in these materials—in particular the

factors which promote high electrical conductivity—it is quite

surprising that the knowledge base regarding the effect of

conjugated cross-links is still rather limited. However, the

experimental examples compiled in this review demonstrate

that conjugated polymer networks with well-defined chemical

structures can be synthesized and processed by a variety of

approaches. The available data indicate that this structural

motif can have significant benefits for the electronic commu-

nication between chains if the polymers are carefully designed.

Future work in this area may further exploit this potential and

lead to the next generation of higher performance organic

semiconducting materials.

Acknowledgements

I thank Drs. D. Knapton and A. Kokil for helpful suggestions

and comments and for proof reading this manuscript. I also

acknowledge fruitful and stimulating collaborations in the

arena of cross-linked PPEs with F. Bangerter, PD Dr.

W. Caseri, E. Hittinger, C. Huber, M. Kinami, Dr. A.

Kokil, C. Rademaker, Dr. I. Shiyanovskaya, Prof. Dr. K.D.

Singer and P. Yao. The related work conducted in my group

has been made possible through generous financial support

from the Case Presidential Research Initiative, the Case School

of Engineering, DuPont (Aid To Education Award, Young

Professor Grant), the Goodyear Tire and Rubber Company,

the Hayes Investment Fund, the National Science Foundation

(NSF DMR-0215342) and the Petroleum Research Fund

(ACS-PRF 38525-AC).

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